1. Field of the Invention
This invention relates to a pressurized cable termination seal and methods of making such a seal, and, more particularly, to an end seal for a gas pressurized cable, where the cable is connected to a terminal, to prevent the escape of gas notwithstanding the exposure of the end seal to a wide range of temperature cycling.
2. Prior Art
Aerial, pulp-insulated conductor cable which is gas-pressurized to protect the cable against moisture, is connected to a telephone distribution system through a short length of cable which is referred to as stub cable and which includes a plurality of individual plastic-insulated conductors enclosed in a plastic jacket. The distribution system includes a telephone cable terminal which is mounted, for example, on a pole, and which includes a plurality of terminal posts and associated protector units encapsulated in a dielectric block mounted inside a weatherproof cabinet having a plurality of entrance ports for drop wires which are run between the terminal posts and the subscribers' premises.
The short length of stub cable has its jacket removed from an end portion which extends through a metal nipple that is supported in an opening in the cabinet. A portion of the nipple on the exterior of the cabinet engages the outwardly facing surface of an end section of the stub cable jacket which, in general, is a plastic material. A portion of the nipple is crimped about the stub cable jacket to prevent gas leaks. However, the engagement of the crimped portion of the nipple with the cable jacket is seldom continuous, thus leaving openings through which gas from the cable can escape.
The insulated conductors in the unjacketed portion of the stub cable extend from the crimped portion of the nipple through the uncrimped portion, and into the cabinet where they are connected to the terminal posts. The insulated conductors are encapsulated in a plugging compound such as, for example, a thermosetting resin, which generally fills the nipple and thereby engages the inwardly facing surface of the nipple.
Since these cable terminals are usually mounted in the open, they are exposed to temperature cycling which is simulated tests may range from -40.degree. F. to 140.degree. F. The thermosetting resin which encapsulates the individual plastic-insulated conductors where they emerge from the jacketed cable and extend into the housing, and which forms an end dam to prevent the escape of gas from the cable, is characterized by a coefficient of expansion which is substantially different from that of the metal nipple. As a result of the difference in coefficients, the thermosetting resin does not move together with the metal nipple during the temperature cycling, but in cold weather, for example, shrinks more than the nipple. This causes gas leak paths to be formed between the thermosetting resin and the nipple or the jacket, or both, notwithstanding the nipple being crimped about the jacket.
The leakage problem is aggravated by the relatively poor bond between the thermosetting resin and the plastic of which the jacket is made, typically polyethylene, at their interface within the crimped section of the nipple. This leads to the beginning of leak paths, which if not blocked by an effective seal, continue through to the atmosphere. Although the use of polyvinyl chloride (PVC) as a jacketing material would result in a better bond with the thermosetting resin, PVC tends to balloon out under pressure and is not used in gas-pressurized cables.
In the past, where lead jacketing was used, gas leakage was not a problem since the external peripheral edge of the nipple was soldered to the lead jacket of the stub cable. Since the introduction of the plastic jacket, one solution to the problem of gas leakage between the jacket and the nipple has been to run a short length of stub cable, having a lead jacket, between the cable terminal and a splice case wherein it is spliced to a plastic-jacketed cable and to solder the nipple which extends into the cable terminal cabinet to the lead jacket of the stub cable. This is an unsatisfactory solution because of the decreasing availability of lead cable, the complexity of the arrangement, and the undue expense which is involved in the step of soldering.
Solutions to the problem of providing an effective pressurized cable termination seal have been proposed in the prior art. H. Fukutomi, K. Ogawa and J. Egashira in a paper entitled "Prefabricated Pressure Dam for Telephone Cable", which appears on pages 140-144 of the Proceedings of the 20th International Wire and Cable Symposium held on November 30 - Dec. 2, 1971, proposed that a thin pipe of resilient material be placed over a conductor-exposed section of cable after which a plugging compound is injected through a slit in the pipe and cured. In another publication, "Development of Cable with Gas - Stoppage Dam by Polyethylene Mold Process" which was authored by M. Azuma, Y. Oishi, K. Fuse and M. Oda and which appears at pages 312-315 of the Proceedings of the 25th International Wire and Cable Symposium held November 16-18, 1976, the problems of sealing gas pressurized cables are recognized and solved by a process which includes the steps of preheating the cable, injecting a plastic material which is the same as that of the jacket and conductor insulation, pressuring and cooling. This process requires substantial investment in facilities, requires precise control of temperatures, and results in a substantially enlarged cross section in order to withstand the flexing of the cable at its junction with the cabinet.
So far as is known, the prior art in conductor seals does not provide a solution to the problem of preventing gas leakage between a thermosetting resin and an enclosing nipple. U.S. Pat. No. 3,829,546, shows a molded bushing which is sealed to an electrical conductor to prevent leakage between the bushing and the conductor by an elastomeric coating on the bushing and by a coating of a thermosetting epoxy resin which adheres to the bushing, the conductor and the elastomeric material. In U.S. Pat. No. 3,113,284, a conductive sleeve is crimped to a terminal after which an outer casing filled with liquid epoxy resin is disposed about the sleeve and the casing filled with liquid epoxy resin to embed the connector. The treatment of individual conductors as disclosed in either of these patents would result in a cable terminal seal having a large cross section, would be difficult and expensive to install in a multiconductor cable, and further an outer casing made of epoxy resin could not be crimped about the cable jacket.
The known prior art has not addressed itself to the problem of effectively sealing a gas pressurized multiconductor, plastic-jacketed cable at its connection to a cable terminal to prevent gas leakage when subjected to temperature cycling.